The present invention relates to a combustor transition piece for flowing combustion products between a turbine combustor and a first stage nozzle and particularly relates to a transition piece having dilution holes to aid in dilution mixing and promoting emissions reduction.
It is well known that air-polluting emissions are typically produced in gas turbines burning conventional hydrocarbon fuels. Those emissions are usually oxides of nitrogen, carbon monoxide and unburned hydrocarbons. It is also well known that oxidation of molecular nitrogen is dependent upon the temperature of the hot gas stream produced by the turbine combustor and which hot gas stream flows through a transition piece to the first stage nozzle. The residence time for the reactants at these high temperatures is also a factor in the production of the undesirable emissions.
Various concepts have been proposed and utilized to maintain the reaction zone temperatures below the level at which thermal NOx is formed or by reducing the residence time at high temperatures such that there is insufficient time for the NOx formation reaction to go forward, or both. One method of reducing the temperature of the reactants in the combustor is to provide a lean mixture of fuel and air prior to combustion. Thus, dilution air is oftentimes provided within the combustion liner to absorb heat and reduce the temperature rise to a level where thermal NOx is not formed. However, in many cases, and even with lean premixed fuel and air, the temperatures are sufficient to produce undesirable emissions.
Dilution air has previously been provided in the transition piece between the combustor and the first stage nozzle. For example, in a prior art transition piece, two dilution holes have been provided adjacent the outlet of the transition piece at a location close to the first stage nozzle. However, undesirable emissions remain a problem, notwithstanding various prior proposals to reduce those emissions.